Note: Descriptions are shown in the official language in which they were submitted.
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MITIGATING THIEF ZONE LOSSES BY THIEF ZONE PRESSURE
MAINTENANCE THROUGH DOWNHOLE RADIO FREQUENCY
RADIATION HEATING
FIELD OF THE INVENTION
[0003] The present invention relates generally to methods and
systems for mitigating thief
zone losses during heavy oil recovery by thief zone pressure maintenance
through downhole
radio frequency radiation heating.
BACKGROUND
[0004] The production of hydrocarbons from low mobility reservoirs
presents significant
challenges. Low mobility reservoirs are characterized by high viscosity
hydrocarbons, low
permeability formations, and/or low driving forces. Any of these factors can
considerably
complicate hydrocarbon recovery. Extraction of high viscosity hydrocarbons is
typically
difficult due to the relative immobility of the high viscosity hydrocarbons.
For example, some
heavy crude oils, such as bitumen, are highly viscous and therefore immobile
at the initial
viscosity of the oil at reservoir temperature and pressure. Many countries in
the world have
large deposits of bitumen oil sands, including the United States, Russia, and
various countries
in the Middle East. The world's largest deposits, however, occur in Canada and
Venezuela.
Oil sands are a type of unconventional petroleum deposit. The sands contain
naturally
occurring mixtures of sand, clay, water, and a dense and extremely viscous
form of petroleum
technically referred to as "bitumen," but may also be called heavy oil or tar.
Indeed, such
heavy oils may be quite thick and have a consistency similar to that of peanut
butter or heavy
tars, making their extraction from reservoirs especially challenging. Due to
its high viscosity,
these heavy oils are hard to mobilize, and they generally must be made to flow
to produce and
transport them. Indeed, such heavy oils are typically so heavy and viscous
that they will
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not flow unless heated or diluted with lighter hydrocarbons. At room
temperature, it is much
like cold molasses.
[0005] As used herein, the term, "heavy oil" includes any heavy
hydrocarbons having
greater than 10 carbon atoms per molecule. Further, the term "heavy oil"
includes heavy
hydrocarbons having a viscosity in the range of from about 100 to about
100,000 centipoise
at 100 F, and an API gravity from about 5 to about 22 API; or can be a
bitumen having a
viscosity less than about 100,000 centipoise, and an API gravity less than or
equal to about
22 API.
[0006] Conventional approaches to recovering heavy oils often focus on
methods for
lowering the viscosity of the heavy oil or heavy oil mixture so that the heavy
oil may be
mobilized and produced from the reservoir. Examples of methods for lowering
the heavy oil
viscosity include introducing a diluent to the heavy oil or heating the heavy
oil. Commonly
used thermal recovery methods include a number of technologies, such as steam
flooding,
cyclic steam stimulation, and steam assisted gravity drainage (SAGD), which
require the
injection of hot fluids into the reservoir. A 100 F increase in the
temperature of the heavy oil
in a formation can lower its viscosity by two orders of magnitude.
Accordingly, heating a
formation containing heavy oils can dramatically improve the efficiency of
heavy oil
recovery.
[0007] While these diluent and thermal recovery methods are often effective
at
recovering heavy oils, these methods may fail to be economical under certain
conditions. For
instance, some hydrocarbon reservoirs may be in thermal or fluid communication
with a thief
zone. Thief zones are gas or water pools to which steam, diluent, heat, or
hydrocarbons may
escape. In the example of SAGD-assisted hydrocarbon recovery, a well pair is
used to
develop a steam chamber in the hydrocarbon reservoir that interacts with and
acts to produce
the heavy oil around the steam chamber. If the SAGD steam chamber happens to
establish
thermal or fluid communication with a neighboring thief zone in the vicinity
thereof, steam,
hydrocarbons, or heat will be lost to the thief zone or conversely, the thief
zone gas/liquids
could invade the SAGD steam chamber. The amount of steam lost to a neighboring
thief zone
can render thermal recovery processes uneconomical.
[0008] Where diluent-assisted recovery methods are employed, thief zones
can similarly
rob the producing zone of valuable diluent or otherwise contaminate the
diluent as the case
may be. Because solvent is typically quite expensive, the process economics of
using diluents
are highly sensitive to solvent losses. Thus, as is the case for thermal
recovery processes,
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neighboring thief zones can also render diluent-assisted recovery methods
uneconomical as
well.
[0009] Conventional methods for mitigating losses to thief zones rely on
the injection of
non-condensable gases into the thief zone. In these conventional methods, the
non-
condensable gas is injected into the thief zone with the hope of maintaining
or increasing the
pressure in the thief zone to minimize or eliminate losses to the thief zone.
These
conventional methods however remain fairly new and accordingly, operators have
not
accumulated much experience with these methods. Thus, the predictability of
these
conventional methods remain fairly unpredictable. Another disadvantage of
these
conventional methods is the risk of contaminating the hydrocarbon reservoir
with the injected
non-condensable gases.
[0010] Still another disadvantage of conventional methods is the
contamination that
results to the thief zone itself. In some cases, the thief zone may be another
hydrocarbon-
bearing reservoir (in some cases owned by a different operator). Injecting non-
condensable
gases into another's hydrocarbon-bearing reservoir will result in
contamination of the thief
zone hydrocarbons. Where these hydrocarbons are owned by another operator,
liability for
the devaluation of those thief zone hydrocarbons will be borne by the
contaminator of the
thief zone. Accordingly, avoiding thief zone contamination in such instances
is highly
economically desirable to avoid incurring such liability to other operators.
[0011] Accordingly, enhanced methods for mitigating or reducing thief zone
interactions
are needed that address one or more disadvantages of the prior art, especially
as relating to
thermal and diluent-assisted hydrocarbon recovery techniques.
SUMMARY
[0012] The present invention relates generally to methods and systems for
mitigating
thief zone losses during heavy oil recovery by thief zone pressure maintenance
through
downhole radio frequency radiation heating.
[0013] One example of a method for mitigating thief zone losses during
heavy oil
recovery by thief zone pressure maintenance through downhole radio frequency
radiation
heating comprises the steps of: introducing a steam assisted gravity drainage
(SAGD) well
pair into a subterranean formation, wherein the SAGD well pair comprises a
producing well
and a steam injection well, wherein subterranean formation comprises a
hydrocarbon
reservoir wherein the hydrocarbon reservoir comprises hydrocarbons;
introducing steam into
the steam injection well to establish a steam chamber in the hydrocarbon
reservoir;
introducing an antenna into the subterranean formation, wherein the antenna is
operable
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connected to an energy source, wherein the subterranean formation comprises a
thief zone,
wherein the thief zone is in thermal communication, fluid communication, or
both with the
steam chamber, wherein the thief zone comprises water; inducing radio
frequency radiation
in the antenna by way of the energy source; allowing the radio frequency
radiation to
propagate into the thief zone to heat at least a portion of the water therein
to form steam to
increase the pressure in the thief zone from a first thief zone pressure to a
second thief zone
pressure, wherein the second thief zone pressure mitigates or eliminates
hydrocarbon or heat
losses to the thief zone that would otherwise occur if the thief zone had
remained at the first
thief zone pressure; and producing the hydrocarbons from the hydrocarbon
reservoir through =
the producing well.
[0014] One example of a method for mitigating thief zone losses by thief
zone pressure
maintenance through downhole radio frequency radiation heating comprises the
steps of:
introducing an antenna into a subterranean formation, wherein the antenna is
operable
connected to an energy source, wherein the subterranean formation comprises a
hydrocarbon
reservoir and a thief zone, wherein the thief zone is in thermal
communication, fluid
communication, or both with the hydrocarbon reservoir, wherein the hydrocarbon
reservoir
comprises hydrocarbons, and wherein the thief zone comprises a thief zone
fluid susceptible
to heating from radio frequency radiation; inducing radio frequency radiation
in the antenna
by way of the energy source; allowing the radio frequency radiation to
propagate into the
thief zone to heat at least a portion of the thief zone fluid therein to
vaporize the thief zone
fluid to form a thief zone gas to increase the pressure in the thief zone from
a first thief zone
pressure to a second thief zone pressure, wherein the second thief zone
pressure mitigates
fluid or heat interaction between the thief zone and the hydrocarbon reservoir
that would
otherwise occur if the thief zone had remained at the first thief zone
pressure; and producing
the hydrocarbons from the hydrocarbon reservoir.
[0015] The features and advantages of the present invention will be
apparent to those
skilled in the art. While numerous changes may be made by those skilled in the
art, such
changes are within the spirit of the invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0016] A more complete understanding of the present disclosure and
advantages thereof
may be acquired by referring to the following description taken in conjunction
with the
accompanying figures, wherein:
[0017] Figure 1 illustrates an example of a system using a radio frequency
radiation to
mitigate thief zone losses in accordance with one embodiment of the present
invention.
[0018] While the present invention is susceptible to various modifications
and alternative
forms, specific exemplary embodiments thereof have been shown by way of
example in the
drawings and are herein described in detail. It should be understood, however,
that the
description herein of specific embodiments is not intended to limit the
invention to the
particular forms disclosed, but on the contrary, the intention is to cover all
modifications,
equivalents, and alternatives falling within the spirit and scope of the
invention as defined by
the appended claims.
DETAILED DESCRIPTION
[0019] The present invention relates generally to methods and systems for
mitigating
thief zone losses during heavy oil recovery by thief zone pressure maintenance
through
downhole radio frequency radiation heating.
[0020] In certain embodiments, methods and systems are provided for
mitigating losses
to thief zones using radio frequency radiation. In one embodiment, a thief
zone may be
situated in a hydrocarbon reservoir or in proximity to the hydrocarbon
reservoir in a way that
poses a risk of losing valuable components from the hydrocarbon reservoir to
the thief zone
during hydrocarbon recovery efforts. Thief zones include any zone that allows
loss of
valuable components from the hydrocarbon recovery process including produced
oil, diluent,
solvent, treatments, heat, water, steam, pressure, and the like. Thief zones
include areas of
the reservoir that cannot be produced or that reduce productivity of the
reservoir indcluding
lean zones, bottom water and the like. Examples of hydrocarbon recovery
processes that may
be employed are diluent-assisted recovery processes or thermal recovery
processes such as
the SAGD process. Not only does the presence of the thief zone pose a risk of
loss of either
diluent, heat, or steam to the thief zone, but valuable hydrocarbons can be
lost to the thief
zone as well.
[0021] One way to mitigate or eliminate these losses to the thief zone is
by maintaining
or increasing the thief zone pressure to alter the driving forces that
motivate losses to the thief
zone. Radio frequency radiation may be used to heat a fluid in the thief zone,
such as water
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for example. By vaporizing the water to steam, pressure is maintained in the
thief zone
decreasing the driving force for losses to the thief zone. In some cases, the
steam generated in
the thief zone may be used to further enhance thermal recovery of the
hydrocarbons.
[0022] Advantages of the enhanced methods and systems described herein
include one or
more of the following advantages: lower cost, higher efficiencies, higher
recovery of
reservoir hydrocarbons, less hydrocarbon contamination, and fewer losses to
the thief zone
(e.g. heat, steam, hydrocarbons, or diluent). Additionally, the methods herein
may also have
the positive effect of introducing heat to the heavy oils to reduce their
viscosity and increase
their mobility, making them easier to recover. Other features, embodiments,
and advantages
will be apparent from the disclosure herein.
[0023] Reference will now be made in detail to embodiments of the
invention, one or
more examples of which are illustrated in the accompanying drawings. Each
example is
provided by way of explanation of the invention, not as a limitation of the
invention. It will
be apparent to those skilled in the art that various modifications and
variations can be made in
the present invention without departing from the scope or spirit of the
invention. For instance,
features illustrated or described as part of one embodiment can be used on
another
embodiment to yield a still further embodiment. Thus, it is intended that the
present invention
cover such modifications and variations that come within the scope of the
invention.
[0024] Figure 1 illustrates an example of a system using a radio frequency
radiation to
mitigate thief zone losses in accordance with one embodiment of the present
invention. In
this example, hydrocarbon recovery process 120 intersects subterranean
formation 105 for
producing hydrocarbons from hydrocarbon reservoir 107. Here, for illustrative
purposes
hydrocarbon recovery process 120 is depicted as a thermal recovery process, in
this case, a
SAGD well pair comprising production well 121 and steam injection well 122.
Other thermal
recovery processes that may be used with the methods described herein include,
but are not
limited to, a cyclic steam stimulation, a vapor extraction, a J-well SAGD, in
situ combustion,
a high pressure air injection, an expanding solvent-SAGD, a cross-SAGD
process, or a
combination thereof.
[0025] Where a SAGD thermal recovery process is employed, steam is
typically
introduced by way of upper steam injection well 122 to mobilize heavy oil in
hydrocarbon
reservoir for production through production wellbore 121. Over time, the
circulation of steam
and condensing fluids establishes steam chamber 124 about hydrocarbon recovery
process
120 in hydrocarbon reservoir 107.
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[0026] As might be expected, expanding steam chamber 124 may come into heat
or fluid
communication with first thief zone 111. Thief zone 111 poses a risk of losses
from
hydrocarbon reservoir 107 to thief zone 111. For example, heat may propagate
through
conduction and/or convection from hydrocarbon reservoir 107 to thief zone 111.
Thief zone
111 comprises a thief zone fluid, in this case, water, which can act as a
significant heat sink
into which sizable heat losses may transfer. In addition to heat losses,
hydrocarbons and/or
steam may also be lost to thief zone 111. Where hydrocarbon recovery process
120 is a
diluent-assisted recovery process, thief zone 111 can rob the hydrocarbon
reservoir of
valuable diluent or solvent as well.
[0027] Energy generator 162 is operably connected to antenna 164 for
generating radio
frequency radiation directed to thief zone 111. The thief zone fluid, in this
case water, is
heated through interaction with the radio frequency radiation and is vaporized
to a thief zone
gas, in this case steam. The steam thus produced maintains or increases the
pressure in thief
zone 111 so as to reduce any driving force that might motivate fluids to
escape to thief zone
111. Additionally, heating thief zone 111 reduces any thermal driving force
that would
otherwise motivate heat transfer from hydrocarbon reservoir 107 to thief zone
111. In this
way, losses to thief zone 111 are thus mitigated.
[0028] Likewise, thief zone 112 poses a similar risk of losses to
hydrocarbon reservoir
107. Unlike thief zone 111, which is situated in the overburden, thief zone
112 is situated in
hydrocarbon reservoir 107 itself. In this example, thief zone 112 is a
depleted steam chamber
remaining from an earlier SAGD thermal recovery process in hydrocarbon
reservoir 107.
[0029] Antenna 164 may be situated in any configuration suitable for
directing radio
frequency radiation to thief zones 111 or 112. In certain embodiments, antenna
164 will
follow all or a portion of injection well 160. Antenna 164 may be oriented
along a horizontal
length, a vertical length, or both in relation to one or more thief zones. In
Figure 1, antenna
164 follows an initially vertically orientation and then, multiple horizontal
branches or
orientations along thief zone 111 and thief zone 112. In certain embodiments,
antenna 164
may be situated above hydrocarbon reservoir 107 in the overburden in or
adjacent to thief
zone 111.
[0030] In certain embodiments, the frequency of the generated radio
frequency radiation
is from about 30 kHz to about 300 GHz. Indeed, any suitable frequency for
interacting with
the fluids contained in the thief zone may be used with the instant invention.
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[0031] __ It is recognized that although the methods described herein are with
reference to
water as the thief zone fluid, the thief zone fluid may be any fluid capable
of interacting with
and susceptible to heating by the generated radio frequency radiation.
[0032] The methods contemplated herein may further comprise the step of
determining
an optimal frequency or range of frequencies of the generated radio frequency
radiation for
heating the reservoir hydrocarbons. In this way, an optimal radiation
frequency may be
determined that maximizes energy transfer to the hydrocarbons for a given
energy input.
[0033] Although the examples depict one injection well and one production
well, it is
understood that the methods described herein could be applied to any number of
injection
and/or production wells, including typical circular drive patterns such as the
five-spot, seven-
spot, and nine-spot patterns. In certain embodiments, it may be desired to a
single well for
both injection (e.g. air injection and introduction of radio frequency
radiation) and
hydrocarbon production. Further, it is recognized that the term "mixture" as
used herein also
refers to non-homogeneous mixtures.
[0034] __ Although these examples depict any thief zone, they are applicable
to a lean zone
or bottom water. Lean zones are thin zones with high water saturation within
the targeted
hydrocarbon formation that can act as thief zone during production. Bottom
water is a highly
water saturated zone below the targeted hydrocarbon zone. Pressure, steam,
heat, produced
oil or combinations thereof may be lost to a lean zone or bottom water. Loss
may be
prevented in a lean zone or bottom water by inserting an antenna in the lean
zone or bottom
water to heat the water, when the water is sufficiently heated it will
increase pressure and/or
generate steam. In one example a lean zone is identified during production of
heavy oil from
a SAGD well, an antenna is placed within the lean zone, the water is heated
above boiling
point to release steam until the pressure in the lean zone is raised
sufficiently to prevent loss
of steam, heat, or produced oil into the lean zone. In another embodiment, an
antenna is
placed below a SAGD reservoir in a bottom zone, during production if the
pressure is
reduced, the bottom water is heated using the antenna. Once the water is
sufficiently heated
the pressure in the bottom water will increase and prevent loss of heat, steam
or produced oil
into the bottom water.
[0035] It is recognized that any of the elements and features of each of
the devices
described herein are capable of use with any of the other devices described
herein without
limitation. Furthermore, it is recognized that the steps of the methods herein
may be
performed in any order except unless explicitly stated otherwise or inherently
required
otherwise by the particular method.
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[0036] Therefore, the present invention is well adapted to attain the ends
and advantages
mentioned as well as those that are inherent therein. The particular
embodiments disclosed
above are illustrative only, as the present invention may be modified and
practiced in
different but equivalent manners apparent to those skilled in the art having
the benefit of the
teachings herein. Furthermore, no limitations are intended to the details of
construction or
design herein shown, other than as described in the claims below. It is
therefore evident that
the particular illustrative embodiments disclosed above may be altered or
modified and all
such variations and equivalents are considered within the scope and spirit of
the present
invention. Also, the terms in the claims have their plain, ordinary meaning
unless otherwise
explicitly and clearly defined by the patentee.
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